1. Field Of The Invention
The present invention relates to a method and apparatus for measuring the sound speed of drilling fluid for use in accurately calculating the stand-off distance between a drillstring and the borehole wall while drilling subterranean oil and gas wells.
2. Description Of The Related Art
Apparatus for measuring the inner diameter of a borehole is well known in the art, where a borehole is a well bore drilled into the earth. Until recently, sonde-type devices were commonly used to make such measurements, where a sonde is a wireline device lowered into the borehole after drilling operations have ceased. Borehole measurements using sonde devices is very time consuming and costly, especially considering the fact that drilling operations must cease and the drillstring must be removed. Early sonde devices used mechanical calipers extending from the sonde to contact the borehole wall to measure the borehole diameter. There are several disadvantages associated with the use of mechanical devices, including significant maintenance, frequent failures and breakdowns and a high incidence of inaccurate measurements. Mechanical devices may also cause damage to steel casing of the well.
More recently, ultrasonic acoustic devices have been developed for measuring various parameters, including borehole diameter and stand-off distances in a borehole. At first, the acoustic devices were mounted on a sonde and lowered into the borehole, so that drilling operations still had to be interrupted while the measurements were being made. Also, the sonde had to be mounted with several acoustic devices around the circumference of the sonde for making the appropriate measurements, since the sonde typically did not rotate. Furthermore, the sonde devices with multiple acoustic devices could be very complex and costly.
Subsequently, measurement-while-drilling (MWD) techniques have been developed to overcome the disadvantages of sonde devices. MWD measuring tools typically take advantage of acoustic devices and transducers, where these devices are mounted on or within one or more MWD drill collars provided on the drillstring. The drillstring includes a series of drill pipe serially linked to a series of drill collars. The drill collars are connected to a drill bit used to drill the borehole. The primary function of the drill collars is to provide a downward thrust or weight on the drill bit. Since the stand-off measurements were made while the drillstring is in the borehole, the measurements were typically made of the stand-off distance between the drillstring and the borehole wall. The measurements may be stored and later retrieved, sent immediately to the surface using other communication techniques, or concurrently used in data analysis apparatus.
An acoustic device transmits an acoustic pulse towards a surface, such as the borehole wall, where the acoustic pulse reflects off the surface and is detected by an acoustic receiver. Often, the acoustic device is an acoustic transducer or transceiver capable of transmitting, detecting and receiving acoustic pulses. The elapsed time between the transmission and reception of the pulse, referred to as the round-trip transit time (RTT), is used to calculate or derive the distance of travel. The relative position of the surface is half this distance.
While the use of acoustic devices have facilitated MWD, the primary problem is the determination of the sound speed of the drilling fluid present in the borehole. The drillstring is immersed in the drilling fluid, where the drilling fluid is preferably a special mixture of clay, water and chemical additives pumped downhole through a center bore of the drillstring during drilling operations, out of the drill bit and upwardly through the annulus to return to surface drilling equipment. The drilling fluid cools the rapidly rotating bit, lubricates the drillstring if it is rotating in the well bore, carries rock cuttings to the surface and serves as a plaster to prevent the borehole wall of the borehole from crumbling or collapsing. The drilling fluid also provides the weight or hydrostatic head to prevent extraneous fluids from entering the borehole and control downhole pressures that may be encountered.
In general, the sound speed of the drilling fluid is not known and depends upon the type of drilling fluid being used, the percent of solids existing in the fluid, the salinity and density of the fluid, and the borehole temperature and pressure. Some techniques estimate the sound speed based on theoretical considerations, or otherwise use a measurement of the sound speed taken before or after the drilling operations. However, sound speed values of known drilling fluids vary from less than 1,100 meters per second (m/s) to greater than 1,800 m/s. Also, conditions in the borehole change during drilling operations, which directly modify the drilling fluid and its sound speed characteristics. Therefore, an accurate determination of the sound speed of the drilling fluid is considered necessary for an accurate acoustic stand-off measurement when using acoustic measuring devices.
Of course, the most direct method known for measuring the sound speed is to measure the RTT of a pulse which travelled through a known distance. Establishing the known distance then becomes the primary goal to solving the sound speed measurement. So far, the only reliable way of establishing a known distance for MWD type measuring apparatus has traditionally been to use multiple acoustic devices. One such example is disclosed in U.S. Pat. No. 4,665,511 to Rodney, et al. The apparatus disclosed in Rodney includes at least one acoustic transceiver disposed within a section of the drillstring. A second acoustic receiver is disposed longitudinally along the drillstring at a selected distance away from the transceiver, where the receiver detects a portion of each acoustic pulse generated by the transceiver. The difference in travel time between the pulse sensed by the second receiver and the pulse sensed by the transceiver is intended to be determinative of the acoustic velocity of the drilling fluid through which the pulses have propagated. This is based on the assumption that the difference in distances of the travel paths of the respective pulses is known.
The primary problem with a device according to Rodney is that the path through which each acoustic pulse travels is not well-defined. Due to the rather coarse or jagged surface of the borehole wall, the highly directional nature of acoustic devices and the high incidence of refracted acoustic pulses, measurement is difficult if not indeterminate. In these conditions, there are several paths that each pulse may travel, so that the receiver may detect multiple reflections. Thus, it is not readily known which reflection represents the desired, predetermined path. Furthermore, even if accurate measurements are acquired, the use of multiple devices significantly increases the cost of the overall system. The cost includes one or more additional acoustic devices as well as the cost of the mounting apparatus for each device. Also, a drill collar appropriately fashioned for mounting multiple devices in appropriate locations is required, and each device must be connected to control circuitry. U.S. Pat. No. 4,979,151 to Ekstrom et al discloses an acoustic stand-off measuring system for a wireline logging system wherein a second transducer unit is used to obtain a direct measurement of the sound speed of the drilling fluid. The pending application Ser. No. 08/104,433 (Kevin S. Warner) filed Aug. 8, 1993 and assigned to the assignee of this disclosure teaches the use of a second transducer into to directly measure the sound speed of the drilling fluid in a MWD embodiment.
Other techniques have been tried using a single transceiver. An example is U.S. Pat. No. 5,130,950 to Orban et al which teaches the use of a single transducer to measure standoff in an MWD embodiment wherein the sound of the drilling fluid is obtained from tables of sound speed as a function of the temperature and pressure of the drilling fluid. None of these techniques to date has been successful since they either ignore the sound speed of the drilling fluid or estimate the sound speed based upon theoretical values or measured values taken before or after drilling operations. Obviously, these devices have limited accuracy.
Thus, it is desirable to measure the stand-off distance and sound speed of drilling fluid as accurately as possible and during drilling operations, using the simplest and most cost-effective system as possible. It is preferable if only a single transceiver or transducer is used while still obtaining accurate measurements.